Define Data Collections used in the spec

[benjie]

The spec uses the terms "set", "list" and "map" in many places. This PR defines these terms and their conformance requirements.

This is a change extracted from #1063 as discussed at last night's WG meeting

The next PR in this stack is:

• https://github.com/graphql/graphql-spec/pull/1092

cc @graphql/tsc

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[benjie]

This was split from RFC2 PR #1063, so I have marked it as RFC 2 itself.

[benjie]

Oops, apparently I pushed to a branch on the spec itself rather than my fork, hence this was out of date. Fixed.

[benjie]

Moved to a branch on the GitHub spec repo directly: https://github.com/graphql/graphql-spec/pull/1102